xfs: use tr_growrtalloc for growing rt files

This is a regression from the following commit:

3d3c8b52 xfs: refactor xfs_trans_reserve() interface

Use the tr_growrtalloc log reservation for growing the
bitmap/summary files.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>